Probing the Magnetism of Topological End States in 5-Armchair Graphene Nanoribbons

Abstract

We extensively characterize the electronic structure of ultra-narrow graphene nanoribbons (GNRs) with armchair edges and zig-zag termini that have 5 carbon atoms across their width (5-AGNRs), as synthesised on Au(111). Scanning tunnelling spectroscopy measurements on the ribbons, recorded on both the metallic substrate and a decoupling NaCl layer, show well-defined dispersive bands and ingap states. In combination with theoretical calculations, we show how these in-gap states are topological in nature and localised at the zig-zag termini of the nanoribbons. Besides rationalising the driving force behind the topological class selection of 5-AGNRs, we also uncover the length-dependent behaviour of these end states which transition from singly occupied spin-split states to a closed-shell form as the ribbons become shorter. Finally, we demonstrate the magnetic character of the end states via transport experiments in a model two-terminal device structure in which the ribbons are suspended between the scanning probe and the substrate that both act as leads. TOC:

Bottom-up Fabrication and Atomic-scale Characterization of Triply-linked, Laterally π-Extended Porphyrin Nanotapes

“…Finally, the state at 0.3 V can be assigned to the HOMO which is delocalized over the central part of the tape (Figs. 4f and 4g), suggesting that the Por 5 NT is (positively) charged on NaCl/Au(111), as has also been observed for short 5AGNRs on NaCl/Au(111) 34 . However, because the SOMO lies lower in energy than the HOMO (Fig.…”

“…However, the termini of AGNRs with a transverse width of 7 carbon atoms (7AGNR) clearly display spin-split end states after being transferred onto a thin insulating film ("decoupling layer") 33 . 5AGNRs are also predicted to host localized magnetic moments at their termini, and a zerobias resonance in STM-based transport spectra has indeed been detected for 5AGNRs with one of their termini located on a decoupling layer 34 . Furthermore, localized end states featuring zero-bias resonances have been observed for an ethynylene-bridged anthracene polymer 35 .…”

“…We have reported a facile on-surface route towards the preparation of meso-meso, β-β, β-β triply- which go beyond the actively explored pure carbon-based 1D magnetic systems [34][35][36] .…”

“…Due to electron correlations, the unpaired electrons at the two ends of the Por NTs are expected to result in spin-polarized singly occupied and unoccupied molecular orbitals (SOMO and SUMO, respectively) 37 . To prevent orbital hybridization with the underlying metal surface (and thus to enable probing of intrinsic electronic properties), a common strategy is to intercalate a thin insulating film between the molecules and the metal surface 33,34,37 . Following the formation of the Por NTs, we therefore deposited a submonolayer of NaCl onto the samples, which led to bilayer islands of (001)terminated NaCl on parts of the Au(111) surface.…”

Electronic decoupling of polyacenes from the underlying metal substrate by sp3 carbon atoms

“…Herein we report the decoupling of polyacenes from an underlying Ag(001) surface by exploiting the non-planarity of the organic backbone imposed by sp 3 -type functional groups. Interestingly, this approach could easily be extrapolated to other organic species and thereby allow important advances in timely research fields like carbon-based magnetism [28][29][30][31][32][33][34] .…”